1. Field of the Invention
The present invention is directed to a magnetic tape drive system. More specifically, the present invention is directed to a motor, capstan and tone wheel assembly having means for cancelling intrinsic errors of the tape drive system.
2. Description of the Prior Art
Conventional magnetic tape drive systems for driving a magnetic recording tape have used various means for controlling the speed of tape in order to maintain the speed at a constant predetermined level, e.g., to control a tape playback speed to be identical with a tape recording speed. For example, the tape can be prerecorded with a clock, or sync, track at the time the data is being concurrently recorded in the adjacent recording track. When the tape is used in a data-reproducing system to playback the data recorded thereon, the sync track is separately read by a control system arrangement commonly referred to as a "sync-off-tape" type of tape playback speed control. In such a prior art system, the signal from the sync track is used in a feedback control to control the speed of the motor driving the tape during the playback operation and, hence, to maintain the playback speed of the tape at the same speed used during the recording process. Thus, the intrinsic tape drive system errors created by capstan eccentricity, bearing noise, etc. are minimized since the control signal comes from the tape itself and is affected by the same tape drive system errors. Since the mechanical errors are included in the aforesaid feedback controls, the servo system, if it has sufficient gain and band width, is able to cancel all of these intrinsic drive system errors. An example of such a prior art tape drive control system is shown in U.S. Pat. No. 3,789,379 of Ivars P. Breikss, which was issued on Jan. 29, 1974.
On the other hand, in another prior art arrangement, i.e., in a so-called "sync-off-tach" tape drive system, a tone wheel is fastened to the shaft of the drive motor used to drive the tape capstan, and a sync signal is derived from a sensor operatively associated with the tone wheel in a so-called tachometer assembly. In this case, the tape drive errors due to capstan eccentricity, bearing noise, mechanical inperfections in the motor and tone wheel, etc. are outside the feedback control loop and, hence, introduce errors in the playback speed which are reflected as errors in the playback data. Since the "sync-off-tape" type servo control system is often not practical inasmuch as it involves the use of a data-recording track and requires a custom prerecording of a clock track, the "sync-off-tach" type control system is more commonly used and is, as mentioned as above, subject to the intrinsic errors in the speed of the driven tape. An example of a prior art tape drive system for cancelling intrinsic errors in a sync-off-tach mode of operation is shown in U.S. Pat. No. 3,648,141 of David W. Scheer which issued on Mar. 7, 1972. In this prior art tape drive system, the intrinsic errors of the drive system for incremental tone wheel positions are recorded in a memory during a recording operation and are utilized during the playback operation to compensate for the same intrinsic errors at each corresponding incremental tone wheel position. However, this prior art compensation technique involves the use of a separate data storage memory, digital-to-analog converters and associated electronics which greatly increases the cost and complexity of a tape transport utilizing such a technique.